Hello guest! Are you an Apistogramma enthusiast? If so we invite you to join our community and see what it has to offer. Our site is specifically designed for you and it's a great place for Apisto enthusiasts to meet online. Once you join you'll be able to post messages, upload pictures of your fish and tanks and have a great time with other Apisto enthusiasts. Sign up today!
I would only keep a pair in that size tank. If it were a 15 then maybe but the females are likely to do battle once they have fry. So it would be best as pairs but you might be able to try it if you load the tank with plants and a bunch of flower pots, like 6-8 of them. I have never put two females in a 10 together that were breeding size. Just be careful about it.
Would it increase a fishes aggression not to have adequate hiding spots and caves to hide in. Will apistogramma borellii breed without any caves to lay their eggs in or will that just make them more aggressive?
To my experience, they do need a cave to breed in. Or at the very least something upside down, like under driftwood, or under a rock. It is best for them to have caves, as they can usually hide in them also.
As for the space, I can't tell with apistogramma, but I've noticed with my Pelvicachromis sacrimontis, and P. pulcher, as the size of the tank went down, the agressivity level went up. in a 5g setup, my P. pulcher would be on the point of killing each other. In a 15g, they would not hurt each other at all.